Wearable activity monitoring devices, so-called “activity trackers” are intended to determine overall activity of a person during his/her daily duties and have become popular lately. There are devices utilizing heart rate measurement, acceleration measurement and satellite positioning systems. Some devices utilize only internal sensors, some receive data from external modules, such as foot pods, heart rate sensors or positioning pods. There are devices that are provided in the form of modules attachable to a chest band or other piece of garment or body part. One class of devices is provided in the form of wristbands (so-called “activity bracelets)” with integral accelerometer for measurement of movements of the users wrist during his or her daily activities and determination of an activity index, which is presented to the user. The activity tracking can also be implemented as a function in a multifunction digital wrist watch, wristop computer or other mobile device, such as a mobile phone.
In particular for people aiming at serious activity tracking for the purposes of fitness monitoring or body weight control, the activity indices given by the existing devices, in particular those based on acceleration only, are often too inaccurate or inconsistent between different usage periods. There is a particular difficulty in determining activity using acceleration sensors located on the wrist of the user, because wrist movement can relate either to ordinary lightweight daily tasks or hard-intensity training, or anything between these extremes. Therefore, as concerns the real physiological effect of the activities done, either too little or too much activity may be registered.
For obtaining a better estimate of real activity, one can try to detect which kind of exercise the user is carrying out. U.S. Pat. No. 7,328,612 discloses a method and apparatus for detecting types of exercise from an acceleration signal using at least two different characteristics computed from the acceleration signal and comparison tables for the characteristics stored in the memory of the apparatus. The method is particularly designed for acceleration signal measured at a lower limb of a person and is not expected to provide equally reliable results for wrist-based measurements, because wrist movement is only loosely related to feet movements. U.S. Pat. No. 5,989,200 disclose a method which computes and utilizes an average of peak accelerations caused by steps.
Measuring and computing a heart rate value is in most cases done continuously, and the measured heart rate data especially when put in context of the activity executed, yields much useful information for analyzing the performance and the abilities of the user of such a device. However, continuous measuring, signaling and computing represents both a technical and a user-experience-related problem of the presently available activity trackers, which the short battery life. This is particularly a problem with optical sensors collecting physiological data, such as the heart rate, from the human body. Thus, there is a need for improved activity tracking devices and methods.